The ability to sense and initiate a response to situations of nitrogen-limitation is essential for bacterial survival. In extensively investigated organisms, the nitrogen-stress response consists of changes in intracellular metabolite levels, post-translational modification of proteins (such as metabolic enzymes) and a transcriptomic response mediated by a global response regulator. However, in mycobacteria the nitrogen stress response has not been comprehensively investigated. In this study mycobacterial nitrogen limiting conditions were optimised and the mechanism of GlnR activation investigated; M. smegmatis GlnR requires a highly conserved aspartate residue (D48), corresponding to a putative phosphorylation site, for function. In addition, a ChIP-seq approach combined with global expression analyses, permitted characterisation of the GlnR mediated global transcriptomic response stimulated during nitrogen.
In M. smegmatis, 52 GlnR binding sites were identified, controlling the expression of at least 103 genes in response to nitrogen limitation. The majority of GlnR regulated genes were involved in nitrogen uptake and nitrogen scavenging. A consensus GlnR DNA binding motif was identified and AC-n9-AC DNA residues shown to be essential for GlnR:DNA binding. In M. tuberculosis 36 GlnR binding sites were identified in nitrogen limitation, however no consensus GlnR:DNA binding motif could be determined. Initial analysis suggests GlnR may be involved in a general stress response in M. tuberculosis, rather than mediating a nitrogen scavenging response as observed in M. smegmatis.
This study provides the first global analysis of nitrogen limitation in mycobacteria and identifies GlnR as the main nitrogen response regulator. From this analysis it appears that the role of GlnR is different in M. tuberculosis compared to M. smegmatis, which may provide key insights into how pathogenic and non-pathogenic species survive nutrient limiting conditions.Open Acces
